Semiconductors, including microprocessors and other integrated circuits (ICs), generate heat during use. Current microprocessors, for example, can emit 50 watts of power or more. The temperature of the microprocessor or IC has a direct impact upon its performance. Empirical studies have shown that the failure rate doubles for every 10° C. increase in the junction temperature (i.e., the temperature of a transistor within the IC).
Unless microprocessors and other ICs are thermally managed during use, they will not operate reliably. Failures include phenomena such as junction fatigue, electromigration diffusion, thermal runway, and electrical parameter shifts. For most uses of a semiconductor device, a proper mechanism for heat dissipation is needed.
Heat may be transferred from the semiconductor by convection, radiation, or conduction. Convection is the transfer of heat by moving air. Radiation is the transfer of heat from one surface to another via electromagnetic waves. Conduction is the transfer of heat between two solids, from a higher temperature object to a lower temperature one. Each of these principles may have a part in the operation of heatsinks.
Heatsinks are devices that attach directly to a microprocessor or other hot surface to enhance heat dissipation from the surface. Heat flows from the surface to cooler air through the heatsink. A heatsink is generally designed with a first surface, for engaging directly with the microprocessor, and a second surface, for contact with the cooler air. The second surface, often formed of a plurality of projections or fins, is designed for maximum surface area, and thus maximum contact with the air, to allow heat to dissipate more quickly. The second surface may also include channels, cooling towers, tubes, cold plates, fans, refrigeration systems, or other embedded features.
Typically, a thermal interface material (TIM) is disposed between the heatsink and one or more microprocessors, known as a processor package, or package. The TIM is a synthetic pad composed of materials such as silicon or zinc oxide, which conducts heat away from the microprocessor and toward the heatsink. The TIM achieves this, in part, by evenly distributing the physical contact between the heatsink and the microprocessor.
During operation of a computer or other processor-based system, the package heats up the TIM, which conducts the heat to the surrounding heatsink. The metal in the heatsink conducts the heat to the fins, channels, tubes, or other embedded cooling elements. Where present, the heatsink fan blows air past the cooling elements to dissipate the heat to the ambient air.
One consideration when designing a heatsink is weight. Although copper-based heatsinks may be preferred over aluminum due to better heat transfer results, copper is a heavier material. A typical copper-based heatsink with a fan, for example, is over 500 grams in weight.
A motherboard or other printed circuit board (PCB) within the computer or other processor-based system typically supports the weight of the heatsink. A larger heatsink thus imposes greater stress on the motherboard than a comparable heatsink of smaller mass. A 450-gram heatsink has been shown to deflect a PCB, which can cause component damage as well as damage to the PCB traces and solder pads. Such chronic stresses can cause cracks or component pullout holes, which may damage the electrical circuitry or signal traces, possibly rendering the computer system defective or inoperable.
Dynamic stresses, such as those caused by shipment and other handling, impose particular constraints on computer systems having high-mass heatsinks. Computer systems are expected to arrive at the customer intact, despite rough handling, for example. The forces resulting from dynamic stresses may damage the motherboards that are supporting the larger heatsinks. Stress cracks, component pullout, solderball stress, or other damage to the electrical circuitry may result from such dynamic loading.
Thus, there is a continuing need for a heatsink mounting system that structurally supports large heatsinks while avoiding damage to the system motherboard during both normal operation and dynamic loading conditions.